Regular grass is OK if it hasn't been treated with a pesticide or recently fertilized. But start them out VERY slowly. Eating too much green grass all at once can cause bloat. Put them out for maybe five minutes the first time, then increase it by a minute or two every two to three days.
Guinea pigs do better in cooler temperatures rather than warmer ones. They can die of heat stroke at anything above 85 degrees. High sixties or low seventies is best.
They'll need to be in a corral of some sort, or you may lose them under a bush or the house or some other impossible-to-get-out-of hiding place. You also need to stay out with them to protect them from birds, cats, dogs, and stinging insects.
They might be happier if you just cut the grass with some scissors and brought it in for them. Many pigs are afraid of the wide open spaces, and won't move unless there's some sort of shelter over them. You could use a large cardboard box with the sides cut out for that, or a small table, or some cloth draped over the end of the corral.